Brake problem
 

Does anyone know why my steering shakes when I brake,I replaced the front discs a short while ago and it seemed to have stoped but now its started again!my rear discs are badly pitted so will replace these,could the rear discs be part of the problem?

Check tie rods & wheel bearings for play

I have the same problem. When I am driving 40mph+ and brake slightly hard, the steering shakes really bad.

At inspection, the mechanic said the car is fine. I got it checked by two mechanics.

But I know it is not supposed to be like that because I drove another Boxster and it did not have that problem.

likely warped brakes. but you'd want to figure out why they're getting warped else you'll be throwing away cash

I had pretty bad steering judder under braking last year and it turned out to be worn, gummed up rotors from insufficient cool down after a track session. I replaced my pads and rotors and added 997 GT3 cooling ducts. So far, so good. Not properly bedding your brake pads can also cause brake judder by not getting an even coating of pad material on the rotors.

Is this a track car or daily driver? What pads are on the front?

Generally when brakes shudder in a Boxster...
 

... it's one of two issues:

a) The rotor has pad transfer. When on the track, if you come back from a session and come to a complete stop while leaving the brake pedal pressed, the extreme heat from the pad/rotors will bond the two together. When you start moving the bond breaks, but there's a thin layer of pad material on the rotor. Next time you brake, every time the pads roll over the embedded material the brakes grab a little more, giving the brakes a shudder.

b) If one or more of the front end's suspension ball joints is badly worn, the shudder will be very marked (almost violent on the track)

I have NEVER seen a Boxster rotor warp.
Rotor warp is thrown around as if it a common thing.
Maybe on a Civic, but not on a Porsche.

Happy Boxstering,
Pedro

Before throwing money at the problem, try this.

When your brakes are reasonable cool, spray them with water.
Leave that on overnight.
In the morning take your car for a drive and work it so that your first braking opportunity is at 30 to 40 mph.
Then stab the brakes hard.
Repeat 2x.

Often times a micro layer of brake residue accumulates unevenly.
The water soaking on the rotors overnight will work through the tiny fissures in the rotor and get underneath the deposits, causing them to come off at hard braking.